n n n Mix all of the marinade ingredients together. Cover the mushrooms with marinade on both sides. (For best results, marinate the mushrooms for at least 2 hours.)n n n n IF USING AN OUTDOOR GRILL: Brush grate with oil. Grill the mushrooms for 5-7 minutes on each side. Use the leftover marinade for basting. n n n n IF USING A STOVETOP: Cook mushrooms in a frying pan covered with a tight lid, on medium-low heat, for about 4-5 minutes on each side. Cooking time may vary depending on the size of the mushroom. If your mushrooms are small, they will need 3 minutes on each side.n n n n While the mushrooms are cooking, toast the buns. You may choose to skip this step.n n n n Spread 1 tsp. of mustard on each bun.n n n n Once the mushrooms are cooked, lift the lid, place the cheese on top, and return the lid, allowing the cheese to melt.n n n n Transfer the mushrooms to the buns, top with fried onions, and assemble the burger.n nn